WiFi calling on Android uses your voice minutes, so you're probably wondering, why do it? Besides the call quality (it's top notch, almost land-line quality) it's a helluva battery saver. It's also the perfect way to make calls outside the T-Mobile service area, without incurring any roaming costs. Speaking of T-Mobile, you must have an active T-Mobile SIM card inserted to use WiFi calling. Other carriers don't have the infrastructure in place to use it. Maybe one day they will, as it's a perfect solution for spotty coverage at home.

If you're interested, and rock a Nexus One with CyanogenMod 7, hit the Android Central forums for the guide to get started. Big thanks to Unforgiven512 for the hard work with this one!